1. Field of the Invention
The present invention relates generally to steppers and more specifically to a production control technique involving the identification of a lens aberration based problem and the corrective modification of a reticle associated with the lens.
2. Related Art
Steppers are highly sophisticated and expensive pieces of apparatus which are used in the production of semiconductor products such as VLSI chips. The most commonly used photolithic process carried out using this type of apparatus are step and repeat and step and scan type operations.
During production, a yield fall out may be observed wherein certain die exhibit certain types of failure. That is to say, within given field a repetitive type of fault or signature may be produced. In most instances, these types of problems can be corrected using conventional measures/techniques. With the individual processes which are carried out, use is made of spot checks of a line/space patterns to check and adjust stepper/process performance. Critical dimension (CD) values are measured by means of Scanning Electron Microscopy (SEM) and then the operation of the stepper, the structure of a reticle which is associated with the process, of a parameter of the process which as been performed and which is responsible in part for the resulting CD""s, is conventionally adjusted in order to bring an errant value or values into line with design requirements.
The above-mentioned stepper devices are, in the case of step-and-repeat lithographic systems, for example, effectively a cameras in which a workpiece is exposed over and over by two dimensional translations of the workpiece beneath the reticle. These steppers use reticles as the object whose image is projected either directly in a 1:1 projection system or through a stepper lens, onto the surface of a workpiece with the reduced image size. In the case of a reduced image system, the pattern transfer from the reticle to the mask on the wafer is accomplished by optically reducing the image to the correct dimensions while simultaneously focusing it onto a specified region of the wafer. After each exposure, the process continues with the wafer being translated to the next desired imaging position where the image is exposed again in the same way. The image is thus sequentially exposed, until the desired regions of the workpiece have been exposed.
Disclosure of the above-type of processes can be found U.S. Pat. No. 4,503,506 issued to Sturges, Jr. on Mar. 5, 1985, and U.S. Pat. No. 4,926,489 issued to Danielson et al. on May 15, 1999.
However, not every element on every die is monitored during the above-mentioned type of quality control, and it can be that some defects are not caught during the fabrication process. Nevertheless, post fabrication testing such as burn in, and the like can provide data pertaining to these type of as yet undetected problems.
One cause of this type of problem can be due to lens aberration. Due to a small imperfection in the formation of the lens, it can occur that the dimensions of an image which is projected onto a layer of photo-resist are either smaller, or otherwise sufficient deviant from those which are correctly generated by the reticle. This type of problem can result in a low yield wherein certain elements of the integrated circuit are defective.
On example of this type of problem are xe2x80x9copensxe2x80x9d where connection between metal lines on different layers has not been established and a test voltage which is applied indicates that current is not able to flow along requisite paths. In this instance, once it is determined that this problem repetitively recurs at each of a series of imprintings, and that it cannot be corrected via the normal adjustments implemented during fabrication, then it can only be assumed that it is a small fault in the lens of the stepper optical system, which is the culprit.
Once it is determined that even the most optimum stepper/process adjustment will not resolve the yield differential problem, there are but few options. One is to not use the xe2x80x9cbadxe2x80x9d stepper which can cause problems with manufacturability, or the yield loss can be tolerated. Due to the cost of the stepper and the lost of productivity, neither of these option are deemed particularly acceptable.
In accordance with the present invention, if a yield signature is observed wherein a repetitive low yield pattern occurs within a given field of dies, and it can be determined to be coming from a particular layer that is being run on a particular stepper, for example, and cannot be corrected by normal adjustments to the stepper operation or process parameters, and it is further determined from results which are obtained with other steppers, that it is the lens of the stepper in question which is the source of the problem, then it is possible, in accordance with concept of the present invention, to reduce the yield loss by modifying the reticle so that, in combination with the localized lens aberration, the resulting CD""s are rectified and meet the required design limitations.
This of course, means that the modified reticle can only be used with one lens arrangement and thus must be dedicated to the stepper in question. Nevertheless, this is far more acceptable than the above mentioned alternatives.
More specifically, a first aspect of the invention resides in a method of manufacturing semiconductor devices using a specified stepper having a predetermined lens and a predetermined reticle, the method comprising the steps of: determining the existence of a yield differential pattern during the production of semiconductor devices using the specified stepper device; determining that the yield differential pattern has characteristics which are consistent with the operation of the stepper which are repetitively located in predetermined locations consistent with repetitive image imprintation during semiconductor fabrication; modifying the reticle which is associated with the lens so as that the effect of the combination of the lens and the modified reticle result in errant fabrication features, which are the cause of the yield differential pattern, being reduced; and dedicating the modified reticle for use with the predetermined lens in the predetermined stepper.
In accordance with this aspect, before proceeding with the reticle modification the steps of: a) effecting adjustments to the operation of the stepper designed to rectify the yield differential following the step of determining that the yield differential pattern has characteristics which are consistent with the operation of the stepper; and b) establishing, before the step of modifying the reticle, that the yield differential is a result of aberration in the lens which is associated with the stepper and that errant fabrication features are generated in given localized locations and are a source of low yield and a cause of the yield differential, are carried out.
In the above, the step of determining the existence of a yield differential pattern includes determining the cyclic recurrence of errant critical dimensions in a predetermined layer.
A second aspect of the invention resides in a manufacturing arrangement for semiconductor devices which includes a stepper having a lens and a reticle, comprising: means for determining the existence of a yield differential pattern during the production of semiconductor devices using the specified stepper device; means for determining that the yield differential pattern has characteristics which are consistent with the operation of the stepper and which are repetitively located in predetermined locations consistent with repetitive image imprintation during semiconductor fabrication; means for modifying the reticle which is associated with the lens, so as that the effect of the combination of the lens and the modified reticle result in errant fabrication features, which are the cause of the yield differential pattern, being reduced, and for dedicating the modified reticle for use with the predetermined lens in the predetermined stepper.
The above arrangement further includes means for effecting adjustments to the operation of the stepper designed to rectify the yield differential prior to modifying the reticle using the reticle modifying means; and means for establishing that the yield differential is a result of an aberration in the lens which is associated with the stepper and that errant fabrication features which are generated in given localized locations and which do not concur with design specifications due to this aberration, are a source of low yield and a cause of the yield differential, and that reticle modification is required.